This invention relates to a laser scanning system and to a laser surface inspection system in which a scanning laser beam is employed for optically detecting microscopic defects in a specularly reflecting surface, such as a silicon wafer.
Silicon wafer discs are used by semiconductor manufacturers as the base substrate in the manufacture of solid state electronic components, such as integrated circuits. The presence of any small defects, such as scratches, pits, or crystal imperfections in the surface, or the presence of surface contaminants such as fingerprints, dust or dirt is highly undesirable and adversely affects the yield of the individual components in production.
Heretofore, silicon wafers have been inspected for flaws or defects by a manual visual inspection technique in which a human inspector visually examines the wafer under an intense light. More recently, automated silicon wafer inspection systems have been developed which are capable of detecting imperfections of a much smaller size than could be detected with the manual visual inspection method. Typically, the automated silicon wafer inspection systems employ a scanning laser beam to inspect the silicon wafers. Laser surface inspection systems of this type have been described for example in an article entitled "A Laser Scan Technique For Electronic Material Surface Evaluation" by D. R. Oswald and D. F. Monroe, published in The Journal of Electronic Materials, Volume 3, No. 1, 1974, pages 225-241; in an article entitled "Silicon/Analyzer Using A He-Ne Laser" by H. J. Ruiz et al, published in The Journal of Electro Chemical Society: Solid State Science and Technology; May, 1974, pages 689-692; in Cuthbert et al U.S. Pat. No. 3,790,287 issued Feb. 5, 1974; in Steigmeier et al U.S. Pat. No. 4,314,763 issued Feb. 9, 1982; and in commonly-owned Alford et al U.S. Pat. No. 4,376,583 issued Mar. 15, 1983.
In these known laser surface inspection systems, a laser beam is traversed across the surface of the silicon wafer and the reflections from the wafer surface are detected and analyzed to provide information about any defects present on the wafer surface. In the absence of defects, all of the light is specularly reflected from the surface. In locations where the beam strikes a surface defect, the light is scattered. The scattered and specularly reflected light may be separately collected and analyzed.
It is desirable in a laser inspection system to use as small a laser spot size as practical and to have it stay in focus as it moves over the area to be inspected. This usually means that the scanner must be some distance from the inspection surface and it must move through a relatively narrow angle.
The silicon wafers used in the manufacture of semiconductor devices have typically had a diameter ranging from about 5 to about 10 centimeters. However, semiconductor manufacturers have recognized that by using larger diameter silicon wafers, (20 centimeters have been shown recently) the semi-conductor devices can be produced in greater quantity and more economically. However, the wafer surface inspection systems presently available are limited in their ability to examine a wide inspection width.
In the wafer surface inspection system described in commonly-owned U.S. Pat. No. 4,376,583, for example, the scan beam B moves through a narrow angle .theta. and in an arc across the inspection surface, in a manner similar to that shown in FIG. 1, such that the scan beam is not always perpendicular to the surface. The reflected beam, R is divergent. For limited inspection widths, this is satisfactory, but the size and geometry of the laser scanning optics limits the scan width which can be covered by this approach.
An objective lens can be added, as shown in FIG. 2 at 16, to form a substantially parallel or collimated scan system. By this arrangement, the bundle of rays that form the spot can be reflected directly back through the objective lens 16 to a point detector 18 located near the scanner. The parallel or collimated scan system is more compact than a diverging scan because the light returned from the surface being inspected can pass back through the scan system to a point detector. This saves the need for fiber optics or other concentrating lenses ahead of the photodetector. In addition, a smaller sensitive area can be used in the detector to detect the entire scan. Also, the entire unit can be closer to the work so that more diffuse light can be gathered by intercepting a larger angle of the light that is scattered, resulting in better gain at the detector.
However, in order to inspect a relatively wide inspection width, the objective lens 16 must be correspondingly large, which is neither practical nor economically attractive.
In U.S. Pat. No. 4,314,763, a laser surface inspection system is disclosed wherein the scan beam is maintained perpendicular to the surface at all times. In this system, the beam is held still and the wafer is rotated under the beam to trace an Archimedes spiral as in playing a phonograph record. This system, however, would be undesirably slow, especially for wide inspection surfaces.
With the foregoing in mind, it is an object of the present invention to provide an improved laser scanning system suitable for use in a laser surface inspection system, and which is especially suited for covering a wide inspection width.
A further object of the invention is to provide a laser scanning system of the type described which is of compact and economical design and construction.
Still another object of the invention is to provide a compact laser scanning system which has a parallel scan pattern and maintains the flying laser spot in focus over the entire inspection width.